This invention relates to heat exchangers, especially for motor vehicles. More particularly, the invention relates to header plates for such heat exchangers, where the header plate is of an aluminum based material and has an outer surface covered with an external coating, and a plurality of through holes, each of which is adapted to receive an end portion of a heat exchanger tube, the tube end portion having a predetermined cross section and having a further external coating.
At the present time, in such aluminum based header plates, the tubes are secured to the header plate, in the region of the through holes in the latter, by welding or brazing. The through holes in this type of header plate are generally made by a press-forming operation which involves removal of some material. The contact between the end portion of a tube, having an external coating, and the header plate, is consequently obtained over the periphery of the through hole into which the tube end portion is introduced. This contact accordingly occurs in the thickness of the wall of the header plate, that is to say precisely in the region where the latter, as a result of the pressing-out operation, no longer has a reliable coating.
The result of this is that the brazed or welded joint between each tube and the corresponding through hole in the header plate gives less than perfect sealing. This can later give rise to leakage and corrosion problems, the consequences of which can be serious.
In addition, because of weight and size limitations which are encountered in the engine compartments of current vehicles, manufacturers are obliged to make use of light and thin materials. Now, thin materials such as are used at the present time do not always have the required mechanical strength characteristics, in particular as regards ability to withstand pressure cycling.
In order to overcome this disadvantage, some header plates are made of an alloy of aluminum and magnesium, which has a better mechanical strength than that of pure aluminum. However, it has become apparent that during the brazing operation some of the magnesium tends to diffuse towards the brazed joint, thus reducing the ability of the surfaces to be brazed together to be wetted. This results in a significant reduction in the effectiveness of sealing in the brazed joint, and this, again, can give rise to leakage and accelerated corrosion.
An object of the invention is to provide a header plate of aluminum based material, of the general kind defined in xe2x80x9cField of the Inventionxe2x80x9d above, which does not have the above mentioned drawbacks.
According to the invention in a first aspect, a header plate of aluminum based material for a heat exchanger, especially for a motor vehicle, the said header plate having an outer surface coated with an external coating, and further having a multiplicity of through holes, each of which is adapted to receive an end portion of a heat exchanger tube of predefined cross section, clad on the outside with an external coating, is characterised in that each through hole is formed by slitting a selected zone of the header plate followed by a configuring operation, the said zone being deformed by press-forming in such a way that the outer surface of the header plate bounds, in the said zone, a collar portion having an internal transverse cross section which is substantially identical to the predefined cross section of the corresponding said tube end portion, so that after introduction of the tube into the through hole in the collar portion, a surface contact is obtained between the respective external coatings of the collar portion and the tube end portion.
In this specification, the term xe2x80x9cslittingxe2x80x9d is to be taken to mean the particular form of piercing that does not involve removal of any material. In the context of the present invention, this slitting operation facilitates the formation of the collar portion, thus enabling an excellent seal to be obtained by brazing.
Preferably, the external coatings of the header plate and tube end portions are of an alloy of aluminum and silicon.
Preferably, the inner face of the header plate, which is opposed to its outer face, is covered with an internal coating which may for example be an alloy of aluminum and zinc, the anti-corrosive properties of which are well known. This enhances the protection of the heat exchanger against corrosion.
In a preferred embodiment of the invention, the header plate is made in an alloy of aluminum and magnesium, the percentage of magenesium being preferably greater than 2%. This leads to a substantial improvement in the mechanical strength of the header plate as compared with a header plate made of pure aluminum.
According to another feature of the invention, an intermediate coating is provided between the outer face of the header plate and its external coating, this intermediate coating being for example in an alloy of aluminum and zinc. The intermediate coating restricts, and can even totally prevent, the diffusion of magnesium towards the external coating, and therefore helps to improve even more the quality of the seal given by the brazed joint.
In a second aspect of the invention, a method of forming the through holes in a header plate according to the said first aspect of the invention is characterised in that it comprises the following steps:
(a) providing a tool which comprises: a punch having a sharp end portion extended by a body having a transverse cross section substantially identical to that of a said tube end portion; and a die formed with at least one die hole having a cross section substantially equal to the transverse cross section of the body of the corresponding punch added to the thickness of the header plate;
(b) locating the die in facing relationship with an inner surface of the header plate, opposed to its outer surface, so that the said die hole lies above a selected zone of the header plate, and then applying the sharp end portion of the punch on the outer surface of the header plate in the selected zone, so as to cause the internal surface of the header plate to penetrate into the die hole; and (c) slitting the said zone so as to rough form a through hole and to cause the material of the header plate, bounding the said through hole, to penetrate into the die hole, so that a collar portion is formed, and then causing the body of the said punch to slide within the corresponding die hole, so that the collar portion and the associated through hole have an internal transverse cross section which is identical to the predefined cross section of the end portion of a tube which is to be fixed in the through hole of the header plate.
According to a preferred feature of the method of the invention, the slitting operation consists of a piercing operation without any removal of material, so as not to damage the external coating in the region of the through holes in the header plate.
Preferably, the method also includes a further step (axe2x80x2) between steps (a) and (b), in which step (axe2x80x2) comprises press-forming the selected zone, firstly on the side of the outer surface of the header plate so as to form on the said inner surface a first offset portion in relief, having dimensions substantially equal to those of the body of the punch, and then, secondly, on the same side as the inner surface of the header plate, so as to form on the said outer surface a second offset portion in relief surrounding at least part of the first offset portion.
Further features and advantages of the invention will appear more clearly on a reading of the following detailed description of some preferred embodiments of the invention, which are given by way of non-limiting example only and with reference to the accompanying drawings.